30x 1W Led Grid, what do I need?

Hi guys,

I want to create a dimmable led grid with 30x 1W Epistar Leds. (12V, 6A, 72W power supply - 3.4V, 350mA LED)

Using this wizard (http://led.linear1.org/led.wiz) it tells me I need 10x 5.6 ohm resistors which needs to handle 686mW. So Can I use 5.6 ohm resistors rated for 2W?

Is this enough to power the leds and dim them (with a mofset) or am I completly on the wrong way? It's all new to me.

Hope you guys can help :)

Hi. No. That wizard is fine for use with ordinary low current leds like 20 or 30mA. But 1W leds are not low current. They draw hundreds of mA.

The problem with high current leds is that they get warm, even hot, so they need heatsinks attached. But the other effect of that temperature rise is that their forward voltage changes. So you need to provide them with a constant current, regardless of their forward voltage. So you need what is called a CC (constant current) driver. In your case you need a CC driver with a control input that allows you to dim them using a PWM signal.

Thanks for helping me out Paul!

I have an aluminium frame (100cm x 4 cm) where I want to glue them to with 2 component epoxy glue and thermal paste. I was thinking the frame itself will act like a heatsink proper enough for 30 led's.

So then I need a CC driver (Can I search for LED driver??) What specs does that LED Driver need for the 30 Leds I described above?

And when I use an LED driver, can I skip the resistors? I just "chain" them in a grid (3x 10 lines) without resistors straight to the driver?

Sounds like your frame will make a good heatsink as long as your glue/paste make a good thermal contact.

Not all led drivers are CC drivers. Many led drivers are designed for led strips. Strips use low current leds, but lots of them, so the small amount of heat each led makes is dissipated over a large area and they get only slightly warm, so don't need a CC driver.

A CC driver will mean you don't need series resistors. The CC driver acts like a self adjusting series resistor.

CC drivers will have a voltage limit which will determine what voltage of power supply you can use and therefore how many leds you can connect in series and so how many CC drivers you will need. You cannot connect leds in parallel to the same CC driver.

So for example let's say you buy a CC driver that can output the 350mA your leds need. It's maximum voltage might be 36V. So you can buy a 36V power supply and connect 10 leds in series to the CC driver (you always need a few more volts than the leds need 10x3.4=34 because the CC driver will drop a few volts). So you will need 3 of those CC drivers for your 30 leds and your power supply will need to provide at least 350mAx3 or for safety 1.5A @ 36V.

nlanaconda: ...it tells me I need 10x 5.6 ohm resistors which needs to handle 686mW. So Can I use 5.6 ohm resistors rated for 2W?

I think 1watt is borderline for resistive current limiting, but as long as you realise the resistors get warm, and it will drop total efficiency somewhat, then I don't see why not. Use strings of three LEDs and one resistor. Leo..

@paulRB

Will something like this constant current driver will be enough, in the description it says "350mA ± 20mA output drive 1-10 pieces 1W LED". So I think this seller means it will be good for max 10 1W led's. So if I use 3 of these for 30 leds (9 LED's for 1 driver) it will work?

@Wawa, I had the same thoughts. But do I need to cool the resistors? If it is rated for 2W I thought it could easily handle ~ 700mW. And how close is it 'borderline'? :P I'm planning to let them burn for 10 hours a day.

Yes, those look ok to me, except for the price. Seems to be the price for 1. I expect you can get 5, perhaps even 10 for that money, on Aliexpress.

nlanaconda: So I think this seller means it will be good for max 10 1W led's. So if I use 3 of these for 30 leds (9 LED's for 1 driver) it will work?

Yes, If you had a 35volt supply.

On a 12volt supply, you can only use THREE LEDs in series. Leo..

Thank for your answers. You've helped me out.

Today I tested 3 leds with a 5.6 ohm 2W resistor and as you guys predicted it works. The resistor gets hot (offcourse).

So by figuring out if I should order LED drivers (and delay my project with a month) or just use the resistors I have 2 questions.

  • How hot may resistors become? If it is rated for 2W and it needs to dissipate 686mW, I'm always safe? The resistor get to hot to touch it longer then 1 second.
  • Are there any other advantages of a led driver? Like is it more energy efficient?

0.7watt in a 2watt resistor is ok as long as it can dissipate the heat and doesn’t melt surrounding things.
Maybe use a joiner block (screw connector strip) on both sides of the resistor bank, so the resistors “float” mid air. If the resistor body doesn’t change colour or starts to smell after a while, you’re ok.

A CC LED driver looses/dissipates about half of the power of a simple resistor (in this case).
So a power saving of about 3-4watt (in this case).
Another advantage is the more constant current. Disadvantage could be the higher price.
Leo…